Overview
We will calculate the value of Rb based on the following circuit:
Calculating the Rb Value
| Precondition | Value |
|---|---|
| Vcc | 12v |
| Vi | 3.3v |
| RL | 155Ω |
| NPN BC337 | |
| Vbe(sat) | 1.2v |
| Vce(sat) | 0.7v |
| hFE (@100mA) | 100 |
...
Vb = Vi - Vbe(sat) = 3.3 - 1.2 = 2.1v
Calculating using Ib=Ic/10
It is common to assume that base current is 1/10 of the collector current.
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Tested with 220 Ω and it worked.
Calculating using hFE
To guarantee that the transistor operates in the saturation region, we multiply the base current by a factor of three.
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Rb = Vb/Ibx3 = 2.1/(0.00078x3) = 897 Ω
Calculating using the online calculator
See reference
hFE=100
Rb = 960 Ω
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Why are these all so different??? I gues it would work from 200 to 1KΩ.
References
| Reference | URL |
|---|---|
| Base Resistor Calculator | https://www.petervis.com/GCSE_Design_and_Technology_Electronic_Products/transistor_base_resistor_calculator/transistor_base_resistor_calculator.html |
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